The primary goal of this study is to examine the dose-response of exercise on long-term weight loss in overweight adult women. Exercise is a key component of behavioral weight loss programs because of its effect on weight loss, body composition, cardiovascular disease risk factors, and other obesity related diseases such as type 2 diabetes mellitus. Despite these benefits, it is unclear whether there is an optimal exercise duration and/or intensity that will enhance long-term weight loss. The current public health recommendation for physical activity is for individuals to participate in at least 30 minutes of moderate intensity physical activity on most days of the week (approximately 150 minutes per week total). However, it is unclear whether this amount of exercise is adequate for long-term weight loss. Moreover, cross-sectional studies have shown that individuals that have been successful at long-term weight loss participate in levels of physical activity that are greater than the current public health recommendations. The researchers hypothesize that adopting higher levels of exercise duration and/or intensity, coupled with a strong behavioral intervention will enhance long-term weight loss. To test this hypothesis, they propose to recruit 200 overweight women and randomly assign them to one of four treatment groups: (1) moderate intensity, moderate 1000cal/wk duration exercise, (2) vigorous intensity moderate duration exercise, (3) moderate 2000cal/wk intensity, high duration exercise, and (4) vigorous intensity, high duration exercise. All subjects will participate in a 24-month behavioral weight loss program, with subjects attending group meetings and receiving periodic telephone contacts from the weight loss program, with subjects attending group meetings and receiving periodic telephone contacts from the interventionists. In addition, to facilitate the adoption of the exercise protocols, all subjects will be provided with a home treadmill. Assessment of body weight will occur at 0, 6, 12, and 24 months and exercise participation will be measured throughout the program. In addition, body composition, cardiorespiratory fitness, cardiovascular disease risk factors, and mediators of behavior change will occur at 0, 6, 12, and 24 months. The investigators believe that the proposed study will lead to improvements in the long-term treatment of obesity.